The present invention relates to a device and method for fatigue testing of materials and in particular relates to a device and method for combined low cycle fatigue and high cycle fatigue testing of materials.
Gas turbine engine fan blades, compressor blades and turbine blades are subjected to a combination of low cycle fatigue and high cycle fatigue stresses in operation of the gas turbine engine. These low cycle fatigue and high cycle fatigue stresses have a detrimental effect on the integrity of the fan blades, compressor blades and turbine blades. The low cycle fatigue (LCF) is a result of the centrifugal force experienced by the fan blades, compressor blades and turbine blades as they rotate about the axis of the gas turbine engine. The high cycle fatigue (HCF) is a result of aerodynamic and other vibration excitation of the fan blades, compressor blades and turbine blades.
The centrifugal force on a fan blade may exert a mean stress of the order of 500 MPa, or more, resulting in low cycle fatigue. The high cycle fatigue fundamental mode frequencies may vary from about 50 Hz for a fan blade to several kHz, for example 2 to 3 kHz, for a high-pressure compressor blade.
The high cycle fatigue damage quickly builds up due to the relatively large number of cycles in relatively short periods of time. The damaging effect of the mechanical cycles is exacerbated by the thermal cycles to which the gas turbine engine is subjected in operation.
In order to design fan blades, compressor blades and turbine blades which are resistant to fatigue, a good understanding of the combination of the steady and alternating stresses a blade may tolerate for any vibration mode that may be excited in operation is required.
The fatigue testing of materials under conditions representative of gas turbine engine operating conditions is difficult to achieve for blade aerofoil shapes and blade root shapes. Conventional low cycle (LCF), high cycle fatigue (HCF) and fatigue crack growth (FCC) have been used to provide mechanical data on simple specimen shapes. Direct comparison between simple specimen shapes and real blades have revealed marked differences in fatigue life. Consequently safety factors, typically 50%, are commonly applied to the fatigue data.
Thus there is a requirement to produce fatigue testing data from specimens whose geometry and state of stress is comparable to real blades in order to aid the design of blades resistant to fatigue or to more accurately determine the working life of real blades.
Accordingly the present invention seeks to provide a novel device for fatigue testing of materials which reduces, preferably overcomes, the above mentioned problems.
Accordingly the present invention provides a device for fatigue testing of materials comprising a frame, first and second clamping means for holding a specimen to be tested, mounting means to mount the first and second clamping means on the frame, the mounting means vibrationally isolating the first and second clamping means from the frame, means to move at least one of the first and second clamping means to apply in operation a low cycle load on the specimen, means to measure the low cycle load, vibration excitation means acoustically coupled to one of the first and second clamping means to apply in operation a high cycle load on the specimen, means to measure the high cycle load, detector means to detect vibration of the specimen and to produce an electrical signal, control means arranged to receive the electrical signal, the control means determining the resonant frequency of the specimen from the electrical signal and sending a signal to the vibration excitation means to maintain the high cycle load at the resonant frequency of the specimen and means to store data of the test.
Preferably the mounting means comprises first leaf spring to mount the first clamping means and a second leaf spring to mount the second clamping means.
Preferably the resonant frequency of the mounting means and first and second clamping means is arranged to be lower than the resonant frequency of the specimen.
Preferably the vibration excitation means comprises an actuator.
Preferably the actuator is arranged to generate frequencies in the range 50 Hz to 5 kHz.
Preferably the actuator is acoustically coupled to the first or second clamping means via a drive rod.
Preferably the actuator is an electrodynamic, piezoelectric or a magnetostrictive actuator.
Preferably there are heating means to heat the specimen.
Preferably the heating means comprises a furnace arranged to surround the specimen.
Preferably electrical insulating means electrically insulate the frame from the specimen.
Preferably there are means to supply an electrical current through the specimen, probes arranged on opposite sides of a crack on the specimen to produce a second electrical signal, means to determine crack growth rate arranged to receive the second electrical signal and to determine the rate of crack growth in the specimen.
Preferably the means to store data stores the life of the specimen to the initiation of the first crack.
Preferably the means to store data stores the life of the specimen to failure
The present invention also provides a method of fatigue testing of materials using a device comprising a frame, first and second clamping means for holding a specimen to be tested, mounting means to mount the first and second clamping means on the frame, the mounting means vibrationally isolating the first and second clamping means from the frame, means to move at least one of the first and second clamping means to apply in operation a low cycle load on the specimen, means to measure the low cycle load, electrical insulating means electrically insulate the frame from the specimen, vibration excitation means acoustically coupled to one of the first and second clamping means to apply in operation a high cycle load on the specimen, means to measure the high cycle load, detector means to detect vibration of the specimen and to produce an electrical signal, control means arranged to receive the electrical signal, the control means determining the resonant frequency of the specimen from the electrical signal and sending a signal to the vibration excitation means to maintain the high cycle load at the resonant frequency of the specimen and means to store data of the test, the method comprising
(a) applying a low cycle load and/or a high cycle load to the specimen,
(b) maintaining the vibration of the specimen at its resonant frequency,
(c) detecting a drop in the resonant frequency of the specimen indicative of the initiation of a crack in the specimen,
(d) stopping the test and locating the crack,
(e) attaching probes to the specimen at each side of the crack, the probes are arranged to produce a second electrical signal,
(f) supplying an electrical current through the specimen,
(g) resuming the test and maintaining the vibration of the specimen at its resonant frequency until failure of the specimen occurs,
(h) determining the rate of crack growth in the specimen from the second electrical signal and/or determining the life of the specimen to failure.
The method may comprise applying tensile load and bending mode vibrations on the specimen.
The method may comprise applying tensile load and torsion mode vibrations on the specimen.
The specimen may be aerofoil shaped.
The method may comprise heating the specimen.
The method may comprise determining the life of the specimen to the initiation of the first crack.
Step (d) may comprise heating the specimen to oxidise and color the surfaces of the crack on the specimen.
Step (b) may comprise maintaining the vibration of the specimen at a predetermined amplitude of vibration.
The method may comprise determining the amount of energy required to vibrate the specimen at the predetermined amplitude of vibrations at the resonant frequency of the specimen.
Preferably the specimen comprises a damping treatment or a damping coating.
The present invention also provides a device for fatigue testing of materials comprising a frame, first and second clamping means for holding a specimen to be tested, mounting means to mount the first and second clamping means on the frame, the mounting means vibrationally isolating the first and second clamping means from the frame, means to move at least one of the first and second clamping means to apply in operation a low cycle load on the specimen, means to measure the low cycle load, electrical insulating means to electrically insulate the frame from the specimen, vibration excitation means acoustically coupled to one of the first and second clamping means to apply in operation a high cycle load on the specimen, means to measure the high cycle load, detector means to detect vibration of the specimen and to produce an electrical signal, control means arranged to receive the electrical signal, the control means determining the resonant frequency of the specimen from the electrical signal and sending a signal to the vibration excitation means to maintain the high cycle load at the resonant frequency of the specimen, probes being provided on the specimen in operation and being arranged to produce a second electrical signal, means to supply an electrical current through the specimen, means to determine crack growth rate arranged to receive the second electrical signal and to determine the rate of crack growth in the specimen and/or determining the life of the specimen to failure.
Preferably there may be means to heat the specimen to oxidise and color the surfaces of the crack on the specimen. The control means may determine the amplitude of vibration of the specimen from the electrical signal and sends a signal to the vibration excitation means to maintain the high cycle load at a predetermined amplitude of vibration. The control unit may determine the amount of energy required to vibrate the specimen at the predetermined amplitude of vibration at the resonant frequency of the specimen. The specimen may comprise a damping treatment or a damping coating.